1. Field of the Invention
This invention relates to a process for melting silicon powder charged into a melting crucible, and a melting crucible for carrying out this process.
2. Description of the Prior Art
As is known, generating melts of silicon powder or dust has proven difficult. In particular, this difficulty arises when fractions having a high proportion of particles with a grain size of less than 1 mm are involved. Such fine-grain powder fractions are produced in large amounts when sawing silicon rods or blocks, crushing or grinding polycrystalline silicon deposited in rod form, or during silicon deposition by the fluidized bed process. German Patent No. 2,706,175 and the corresponding U.S. Pat. No. 4,151,264 teach that a fine silicon dust, which is produced as a waste product during grinding of silicon lumps, can be successfully melted, e.g., in a silicate slag with added copper. However, the silicon attained in this process is unsuitable for applications in which high purity is required if only due to the amount of copper absorbed. Also, additional costs are incurred because of the cost of the slag and the necessity of using a large-capacity melting crucible due to the large amount of material to be melted in addition to the silicon.
In another prior art melting process, a melting crucible is completely filled with silicon powder and this charge is then melted down. Since the silicon powder is less dense than the molten silicon, the melting crucible is only filled to a comparatively small extent after melting the silicon. The crucible must again be designed for a large capacity in order to accommodate the voluminous starting material. Using a large crucible results in an increasingly unfavorable energy utilization and increased crucible corrosion. This is because of crucible overheating which is necessary for producing a melt and which becomes greater with increasing crucible volume.
In addition, it is extremely difficult to charge additional silicon powder into the melt in order to improve the degree of crucible filling. This is because there is considerable liberation or silicon monoxide, which is so vigorous that it frequently can remove the entire content of the crucible. This release is inevitable because of the reaction of the powder to the melt due to the oxide skin generally covering the powder particles.
These difficulties have often been overcome by first eliminating the fine component from the silicon powder which is to be melted. This is an expensive grading step because the fines are difficult to handle, and fines are discarded which, per se, serve as a valuable source of raw material.